1. Field of the Invention
The present invention generally relates to semiconductor structures and, more particularly, to a method to design a low-cost, high-performance charge pump system.
2. Background Description
In semiconductor capacitor structures, oxide reliability concern becomes more and more severe as the oxide thickness is further scaled down. Capacitor dielectric used in xe2x80x9chigh-stressxe2x80x9d circuits could suffer dielectric breakdown if the devices are not properly protected. A general guideline for the dielectric reliability is shown in FIG. 1. If the thickness of the dielectric for a technology is chosen to be 55 nm (the vertical dash line), then for gate or capacitor devices with 10 mm2 area can sustain a voltage V1 (about 2.2 volts) maximum use voltage without having a reliability concern. The smaller the over-all device area, the higher sustainable stress voltage is allowed for the device. On the other hand, the thinner the dielectric layer thickness, the lower the stress it can sustain for a given surface area. In a DRAM (dynamic random access memory) chip, many different kinds of charge pump circuits are required to provide different internally generated voltage levels. These pump circuits usually are equipped with several different sized boost capacitors made by planar capacitors. When the number of the pump circuits are increased, and the size of the boost capacitors used in the pump circuits, the reliability of the capacitor dielectric becomes a concern, especially for those capacitors operated at high voltage levels.
It is therefore an object of the present invention to provide a method to resolve the capacitor dielectric problem in a charge pump system.
It is another object of this invention to significantly reduce the pump size.
It is yet another object of the invention to improve the pump efficiency and capacity.
According to the invention, there is provided a dynamic clamp used in conjunction with capacitors with deep trench capacitors or other high-density capacitors, such as high-k dielectric and three dimensional (3D) stack capacitors. The dynamic clamp is realized using a two stage pump operation cycle such that, during a first stage pump cycle, a middle node of a pair of series connected capacitors is pre-charged to a supply voltage and, during a second stage pump cycle, the middle node is coupled by a boost clock. Thus, at any moment in the pump operation cycle, the voltage across the capacitors is held within a safety range.